1. Field of the Invention
The present invention relates generally to magnetic storage systems, and in particular a spindle motor assembly having increased heat dissipation.
2. Description of the Related Art
Magnetic storage systems may store data on at least one rotatable magnetic disk having concentric data tracks. Data is read from or written to each magnetic disk by a magnetic recording head or transducer, which is supported by a slider. Each transducer may be attached to or integrally formed with a slider, which is resiliently supported by a suspension assembly. During operation of the magnetic storage system, a slider typically supports a transducer above the data surface of a magnetic disk by a cushion of air, referred to as an air bearing surface, generated by the rotating disk.
The transducer/slider/suspension assembly is coupled to an actuator for positioning the transducer over the desired data track during reading or writing operations. The actuator positions the transducer over the desired data track by moving the transducer/slider/suspension assembly across the surface of the disk in a direction generally transverse to the data tracks. The actuator may include a single arm extending from a pivot point, or alternatively a plurality of arms arranged in a comb-like fashion extending from a pivot point. A rotary voice coil motor (vcm), attached to the rear portion of the actuator arm(s), powers the movement of the actuator over the disks.
The magnetic disk or disks are mounted on the hub of a spindle motor assembly. High capacity, high speed magnetic storage systems, such as 7200 rotations per minute (rpm) hard disk drives having a 9 gigabyte capacity, may have a spindle motor assembly that includes a shaft, at least two ball bearing assemblies, a brushless direct current (dc) motor, and a hub. The ball bearing assemblies allow the hub and the magnetic disks to rotate with respect to the shaft when powered by the motor.
As the industry demands larger capacity drives rotating at faster speeds within same form factor (e.g., 1.63xc3x974.00xc3x975.75 inches), the load placed on the motor is increased. The largest contribution to load is viscous dissipation (also referred to as actual drag of air or windage loss), which is highly sensitive to disk speed. Increasing the capacity of the drives without increasing the diameter of the disks may also require that the number of disks loaded onto the hub be increased, resulting in an increased load on the motor.
Most high performance disk drives on the market today have a stationary shaft to increase shaft stiffness and to provide a more sound structure. The increased stiffness provided by a stationary shaft design causes less deflection and better track misregistration (tmr) performance parameters than rotating shafts. Generally, as more load is applied to the motor, the motor is required to do more work resulting in an increase in the power dissipation of the motor. The power dissipated by the motor, and more specifically in stator windings and stator itself, looses heat which primarily travels through the stationary shaft.
Shafts used in today""s motors for high speed disk drives may be made of 300 or 400 series stainless steel. These types of steels are known to have low heat conductivity as compared to other metals such as Aluminum or Copper. Because of the low thermal conductivity of the stainless steel shafts, the shaft prevents most of the heat dissipated from the motor from flowing to the outer surfaces of the spindle motor assembly. As a result, the stator portion of the motor, and the bearing assemblies adjacent to the stator, typically operate at an increased temperature when the spindle motor assembly is operating at higher rotational speeds and/or with increased storage capacity. When operating the bearing assemblies at faster rotational speeds and/or higher temperatures, the bearing assemblies tend to lose lubricant at a faster rate, leading to a shortened bearing assembly life that impacts the overall reliability and performance of the magnetic storage system.
A couple of approaches have been considered to reduce both the motor and the bearing assembly temperatures by increasing the heat transfer out of the motor to the base plate and cover where heat can be removed by convective heat transfer. One approach is to position the stator below the bearing assemblies or external to the bearing assemblies. Although this approach improves the heat sinking of motor losses into the base plate, these designs compromise the bearing span or bearing size and is typically not suited for high performance disk drives. Furthermore, this approach does not fully support the spindle motor hub during a heat shrink disk clamp process.
Another approach is to make the shaft out of a material that is sufficiently thermally conductive. Unfortunately, certain spindle motor design configurations, particularly those suitable for high performance disk drives such as configurations with the stators positioned between the bearing assemblies to provide optimal efficiency and high spindle stiffness, requires a stainless steel shaft having the same coefficient of thermal expansion (CTE) as other parts of the spindle motor assembly. If the CTE of the shaft and other parts of the spindle motor assembly do not match, the bearing preload can not be maintained through temperature changes of the spindle motor assembly. This mismatch may severely impact the performance of the spindle motor assembly. Accordingly, selecting a shaft made of a material that is more thermally conductive than stainless steel may not be suitable for most spindle motors used in high performance disk drives.
In light of the drawbacks of these approaches, there exists a need to improve the heat transfer out of the motor in a spindle motor assembly suitable for high performance disk drives.
It is desirable to increase the heat transfer flowing out of the spindle motor assembly to reduce the operating temperature of the motor and bearing assemblies.
It is also desirable to match the coefficients of thermal expansion between the shaft and various other components of the spindle motor assembly to maintain the preload across the operating temperature range while increasing the heat transfer flowing out of the motor.
A spindle motor assembly is described. The spindle motor assembly includes a bearing assembly, a stationary shaft, a hub, and a motor. The stationary shaft has an outer peripheral surface attached to an inner peripheral surface of the bearing assembly. The stationary shaft is capable of operating as a heat pipe. The motor, positioned between the hub and the stationary shaft, is operable to rotate said hub with respect to a central axis of said stationary shaft. The stationary shaft may include a hollow central portion that is lined with a wick and partially filled with fluid.
A method of assembling a stationary shaft having an integral heat pipe is also described. A stationary shaft having a hollow central region and an open end is provided. The hollow central region is lined with a wick and partially filled with a fluid. The hollow central portion is evacuated. Then, the open end of the stationary shaft is sealed. The stationary end may be sealed by a brazing process.